Production systems are generally installed for relatively long periods, for example 20 years. Throughout the setting thereof and production operations, they undergo external strains and they are stressed by the wave motion, the current, the wind, etc.
The floater is usually anchored statically to the sea bottom by a set of chains or of vertical or oblique taut lines. In both cases, it retains a certain freedom of motion along various axes, that range between some centimeters and some meters for vertical displacements due to the wave motion, well-known as heave, and that can reach several ten meters in the horizontal plane. The amplitudes of the rotations about horizontal axes, known as roll and/or pitch, and about a vertical plane notably depend on the dimensions of the floater, on the anchor means thereof and on the wave motion conditions.
Conventionally, in such installations, riser pipes are on the one hand fastened to a subsea structure resting on the bottom and which generally includes several wellheads and, on the other hand, in direct or indirect connection with the floater. The connecting devices make the risers more or less dependent on the floater and therefore on the displacements thereof. In order that these displacements are bearable by the riser pipe, suspension systems or tensioning systems are generally used to maintain at the top of the riser pipe a relatively constant tension independent of the motions of the floater. Hydraulic tensioning systems can be used.
The prior art describes various layouts whose function is notably to take up the motions of the floaters and to disconnect them from the riser pipes.
For example, patent NO-171,958 describes a floating construction used for production of petroleum products including several riser pipes kept under tension from the wells on the sea bottom to a carrier deck that is mobile in relation to a floater. In this system, the carrier deck is connected to the floater by means of a hauling device for supporting both sways and vertical motions resulting from the motions of the floater. The riser pipes or production risers are connected to the carrier deck by means such as elastic elements for supporting vertical and lateral elastic motions. The use of both systems, the hauling system and the elastic elements, allows to prevent transmission of the motions of the floater to the production riser pipes. However, such an approach requires relatively complex mechanical devices, notably the hauling system, which increase the cost and the complexity of the entire production system.
The basic idea of U.S. Pat. No. 4,702,321 consists in using a caisson of sufficient size to contain the risers and their floats, and thus to shield them from the wave motion. Such a system can induce considerable stresses at the outlet of the guide system.